Semiconductor heterostructure with improved light emission

ABSTRACT

A semiconductor heterostructure for an optoelectronic device with improved light emission is disclosed. The heterostructure can include a first semiconductor layer having a first index of refraction n1. A second semiconductor layer can be located over the first semiconductor layer. The second semiconductor layer can include a laminate of semiconductor sublayers having an effective index of refraction n2. A third semiconductor layer having a third index of refraction n3 can be located over the second semiconductor layer. The first index of refraction n1 is greater than the second index of refraction n2, which is greater than the third index of refraction n3.

REFERENCE TO RELATED APPLICATIONS

The current application claims the benefit of U.S. ProvisionalApplication No. 62/576,700, filed on 25 Oct. 2017, and U.S. ProvisionalApplication No. 62/612,506, filed on 31 Dec. 2017, both of which arehereby incorporated by reference.

TECHNICAL FIELD

The disclosure relates generally to electronic and optoelectronicdevices, and more particularly, to semiconductor light emitting devicescomposed of group III nitride alloys.

BACKGROUND ART

Semiconductor light emitting devices, such as light emitting diodes(LEDs) and laser diodes (LDs), generally include solid state emittingdevices composed of group III-V semiconductors. A subset of group III-Vsemiconductors includes group III nitride alloys, which can includebinary, ternary and quaternary alloys of indium (In), aluminum (Al),gallium (Ga), and nitrogen (N). Illustrative group III nitride-basedLEDs and LDs can be of the form In_(y)Al_(x)Ga_(1-x-y)N, where x and yindicate the molar fraction of a given element, 0≤x, y≤1, and 0≤x+y≤1.Other illustrative group III nitride-based LEDs and LDs are based onboron nitride (BN) and can be of the formGa_(z)In_(y)Al_(x)B_(1-x-y-z)N, where 0≤x, y, z≤1, and 0≤x+y+z≤1.

An LED is typically composed of semiconducting layers. During operationof the LED, an applied bias across doped layers leads to injection ofelectrons and holes into an active region where electron-holerecombination leads to light generation. Light is generated with uniformangular distribution and escapes the LED die by traversing semiconductorlayers in all directions. Each semiconducting layer has a particularcombination of molar fractions (e.g., x, y, and z) for the variouselements, which influences the optical properties of the layer. Inparticular, the refractive index and absorption characteristics of alayer are sensitive to the molar fractions of the semiconductor alloy.

An interface between two layers can be defined as a semiconductorheterojunction. At an interface, the combination of molar fractions isassumed to change by a discrete amount. A layer in which the combinationof molar fractions changes continuously is said to be graded. Changes inmolar fractions of semiconductor alloys can allow for band gap control,but can lead to abrupt changes in the optical properties of thematerials and result in light trapping. A larger change in the index ofrefraction between the layers, and between the substrate and itssurroundings, results in a smaller total internal reflection (TIR) angle(provided that light travels from a high refractive index material to amaterial with a lower refractive index). A small TIR angle results in alarge fraction of light rays reflecting from the interface boundaries,thereby leading to light trapping and subsequent absorption by layers orLED metal contacts.

Roughness at an interface allows for partial alleviation of the lighttrapping by providing additional surfaces through which light can escapewithout totally internally reflecting from the interface. Nevertheless,light only can be partially transmitted through the interface, even ifit does not undergo TIR, due to Fresnel losses. Fresnel losses areassociated with light partially reflected at the interface for all theincident light angles. Optical properties of the materials on each sideof the interface determine the magnitude of Fresnel losses, which can bea significant fraction of the transmitted light. In particular, Fresnellosses associated with high total internal reflection limit efficienciesof light emitting diodes.

SUMMARY OF THE INVENTION

This Summary Of The Invention introduces a selection of certain conceptsin a brief form that are further described below in the DetailedDescription Of The Invention. It is not intended to exclusively identifykey features or essential features of the claimed subject matter setforth in the Claims, nor is it intended as an aid in determining thescope of the claimed subject matter.

Aspects of the present invention are directed to a semiconductorheterostructure with improved light emission. The semiconductorheterostructure of the various embodiments, which can be used tofabricate an optoelectronic device, can include a group III nitridestructure. The optoelectronic device that can be fabricated from thesemiconductor heterostructure of the various embodiments can include alight emitting device. Examples of light emitting devices that can befabricated from any semiconductor heterostructure described herein caninclude, but are not limited to, light emitting diodes (LEDs),ultraviolet LEDs (UV LEDs) and laser diodes (LDs).

In one or more of the various embodiments, light emission is improved byreducing Fresnel reflective losses associated with the total internalreflection (TIR) that occurs at the interfaces between the semiconductorlayers that form the semiconductor heterostructure. In particular, theTIR that occurs between semiconductor layers of the semiconductorheterostructure is substantially eliminated by fabricating layers withan index of refraction that is appropriate for removing the TIR. Ingeneral, the semiconductor heterostructure can include a firstsemiconductor layer having a first index of refraction n1, a secondsemiconductor layer formed over the first semiconductor layer having anindex of refraction n2, and a third semiconductor layer formed over thesecond semiconductor layer having a third index of refraction n3,wherein n1>n2>n3.

In one embodiment, the first semiconductor layer and the thirdsemiconductor layer can take the form of a cladding layer, and thesecond semiconductor layer can take the form of an active region that issandwiched between the cladding layers. To this extent, the lightgenerated from the active region can be outputted from the semiconductorheterostructure at the side of the cladding layer having the refractiveindex n1. In one embodiment, the cladding layer having the refractiveindex n1 can include an n-type cladding layer and the cladding layerhaving the refractive index n3 can include an electron blocking layer(e.g., a p-type blocking layer).

The active region can comprise a complex structure, and in general, cancomprise a laminate of semiconductor layers (e.g., semiconductorsublayers). In one embodiment, the active region can comprise quantumwells and barriers, with the quantum wells being significantly thinnerthan barriers. For example, the quantum wells can have a thickness thatcomprises several nanometers (e.g., a thickness less than 5 nm), whilethe barriers can be at least twice as thick (e.g., as large as ten ormore nanometers). It is understood that the index of refraction of anactive region formed from a laminate of semiconductor layers willinclude an effective index of refraction that accounts for therespective indexes of refraction of the layers of the laminate.

The semiconductor heterostructure can also include a fourthsemiconductor layer having an index of refraction n4. In one embodiment,the fourth semiconductor layer can be formed over the thirdsemiconductor layer such that the fourth index of refraction n4 is lessthan the third index of refraction n3. To this extent, the relationshipof the indexes of refractions of the semiconductor heterostructure thatincludes the aforementioned first semiconductor layer, the secondsemiconductor layer, the third semiconductor layer and the fourthsemiconductor layer can be described by n1>n2>n3>n4. In one embodiment,the fourth semiconductor layer can include a p-type layer. For example,in the embodiment in which the semiconductor heterostructure includes anactive region sandwiched between an n-type cladding layer and anelectron blocking layer, the p-type layer can be formed over theelectron blocking layer. In one embodiment, the p-type layer can includea superlattice having a plurality of interlayers. For example, thesuperlattice of interlayers can include a p-type superlattice having aplurality of barriers.

The indexes of refraction (i.e., n1, n2, n3, n4) of the varioussemiconductor layers in the semiconductor heterostructure and thedesired relationship of the indexes (e.g., n1>n2>n3 and n1>n2>n3>n4) inthe heterostructure can be obtained by selecting appropriate amounts ofgroup III nitride materials (e.g., AlN, GaN) used in the layers. Forexample, the active region can have a composition of a group III nitridematerial that results in the active region having an index of refractionthat is smaller than the first cladding layer (e.g., the n-type claddinglayer) and greater than the second cladding layer (e.g., the electronblocking layer). In a scenario in which the active region comprises aplurality of quantum wells and a plurality of barriers, the barriers canhave a high molar fraction of a group III nitride material with a lowrefractive index, while the quantum wells can have a low molar fractionof a group III nitride material with a low refractive index. Forexample, the barriers can have a high molar fraction of AlN, while thequantum wells can have a low molar fraction of AlN. To this extent, thebarriers will have low indexes of refraction and the quantum wells willhave higher indexes of refraction.

In this manner, the active region will have an effective index ofrefraction that is less than the index of refraction of the n-typecladding layer. It is understood that the molar fraction of AlN withinthe barriers and the quantum wells is chosen to result in an effectiverefractive index of the active region that satisfies the relationshipbetween the layers (i.e., n1>n2). As a result, there will be an absenceof TIR at the interface between the active region and the n-typecladding layer. In this example, the electron blocking layer can have ahigh molar fraction of AlN that results in the index of refraction n3 ofthis layer being smaller than the index of refraction n2 of the activeregion. To this extent, the change in refractive index from n2 to n3results in TIR at the interface between the active region and theelectron blocking layer. With regard to the fourth semiconductor layerthat can be formed with the semiconductor heterostructure, the p-typelayer can have a low molar fraction of AlN.

In order to further improve control of the indexes of refraction of thesemiconductor layers in the semiconductor heterostructure, additionalsemiconductor materials can be added to the semiconductor layers toattain desired indexes of refraction by enhancing electrical and opticalproperties of the layers. In one embodiment, boron nitride (BN) can beadded to any of the semiconductor layers of the semiconductorheterostructure. Adding smaller fraction amounts of BN to a group IIInitride-based semiconductor layer in the semiconductor heterostructurewill not significantly affect the band gap of the semiconductor layer,but such additions can have notable decreases in the index of refractionof the semiconductor layers. For example, a slight addition of BN to AlNand GaN layers can result in significant decreases in the indexes ofrefraction of the semiconductor layers.

In one embodiment, BN can be added to an active region having barriersand quantum wells. In particular, a molar fraction of BN can be added tothe barriers and/or the quantum wells of the active region, with theamount of the BN that is utilized including a molar fraction thatpreserves a targeted radiation wavelength specified for theoptoelectronic device incorporating the semiconductor heterostructure.In one embodiment, a semiconductor heterostructure that includes ap-type layer with interlayers can include a molar fraction of BN in theinterlayers. Similarly, a semiconductor heterostructure that includes ap-type superlattice of barriers can include BN in least one of thebarriers. The control and improvement of the indexes of refraction ofthe semiconductor layers of the semiconductor heterostructure are notlimited to the addition of BN, but can include adding molar fractions ofother materials and compounds such as boron (B) and indium nitride(InN).

The emission of the semiconductor heterostructure of the variousembodiments can also be improved by nano-patterning one or more thesemiconductor layers that form the heterostructure. For example, theactive region can be nano-patterned. In one embodiment, in the scenarioin which the active region includes a laminate of sublayers such asquantum wells alternating with barriers, nano-patterning can be utilizedto form sets of nano domains of stacked semiconductor sublayers (e.g.,quantum wells alternating with barriers). Each nano domain of stackedsemiconductor sublayers can be separated from adjacent nano domains ofstacked semiconductor sublayers by a predetermined spacing. A set ofnano domains of insulating material can be formed about the nano domainsof stacked semiconductor sublayers. For example, each nano domain ofinsulating material can be formed in one of the spacings separatingadjacent nano domains of stacked semiconductor sublayers. In oneembodiment, the set of nano domains of insulating material can have anindex of refraction that is less than the index of refraction of thestacked semiconductor sublayers.

The emission of the semiconductor heterostructure of the variousembodiments can also be improved by utilizing any of a multitude ofother modalities. For example, any of the semiconductor layers can beformed with a roughness surface at the interface with adjacent layers.Roughness at an interface allows for partial alleviation of the lighttrapping by providing additional surfaces through which light can escapewithout totally internally reflecting from the interface. In oneembodiment, an interface between the first semiconductor layer (e.g.,the n-type cladding layer) and the second semiconductor layer or activeregion can include a roughness surface.

Another example of a modality that can be used to improve the emissionof the semiconductor heterostructure of the various embodiments includesapplying an anti-reflective coating to at least one of the semiconductorlayers of the heterostructure. In one embodiment, an anti-reflectivecoating can be applied at a side of the first semiconductor layer (e.g.,the n-type cladding layer) that is opposite to the interface with theactive region.

A first aspect of the invention provides a semiconductorheterostructure, comprising: a first semiconductor layer having a firstindex of refraction n1; a second semiconductor layer formed over thefirst semiconductor layer, the second semiconductor layer including alaminate of a plurality of semiconductor sublayers having an effectiveindex of refraction n2; and a third semiconductor layer formed over thesecond semiconductor layer, the third semiconductor layer having a thirdindex of refraction n3, wherein n1>n2>n3.

A second aspect of the invention provides a semiconductorheterostructure, comprising: a first cladding layer having a first indexof refraction n1; an active region formed over the first cladding layer,the active region including a plurality of semiconductor sublayershaving an effective index of refraction n2; and a second cladding layerformed over the active region, the second cladding layer having a thirdindex of refraction n3, wherein n1>n2>n3.

A third aspect of the invention provides an optoelectronic device,comprising: a semiconductor heterostructure, including: an n-typecladding layer having a first index of refraction n1; an active regionformed over the n-type cladding layer, the active region including aplurality of quantum wells alternating with a plurality of barriers,wherein the plurality of quantum wells and the plurality of barriershave an effective index of refraction n2; an electron blocking layerformed over the active region, the electron blocking layer having athird index of refraction n3; and a p-type layer formed over theelectron blocking layer, the p-type layer having a fourth index ofrefraction n4, wherein n1>n2>n3>n4, and wherein the p-type layerincludes a molar fraction of boron.

The illustrative aspects of the invention are designed to solve one ormore of the problems herein described and/or one or more other problemsnot discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIG. 1 shows a schematic of an illustrative optoelectronic device havinga semiconductor heterostructure according to an embodiment.

FIG. 2 shows a band diagram and an example of a profile of therefractive index for each layer within an illustrative semiconductorheterostructure according to an embodiment.

FIG. 3 shows another view of the band diagram and an example of aprofile of the absorption values of the semiconductor layers of anillustrative semiconductor heterostructure according to an embodiment.

FIGS. 4A and 4B show examples of available band gap energy data forternary and quaternary semiconductor compounds that can be used informing any of the semiconductor layers in a semiconductorheterostructure according to an embodiment.

FIGS. 5A and 5B show examples of available refractive index data forternary semiconductor compounds that can be used in forming any of thesemiconductor layers in a semiconductor heterostructure according to anembodiment.

FIG. 6 illustrates an example of an effect that the addition of boronnitride (BN) has on the profile of the index of refraction ofsemiconductor layers within a semiconductor heterostructure according toan embodiment.

FIGS. 7A and 7B show a cross-sectional view and a top view,respectively, of an illustrative semiconductor heterostructure havingnano-patterned domains according to an embodiment.

FIG. 8 shows an illustrative flow diagram for fabricating asemiconductor heterostructure according to one of the variousembodiments described herein.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the present invention are directed to asemiconductor heterostructure with improved light emission for use withan optoelectronic device. In at least some of the various embodiments,light emission is improved by reducing Fresnel reflective lossesassociated with the total internal reflection (TIR) that occurs at theinterfaces between the semiconductor layers that form the semiconductorheterostructure. In particular, the TIR that occurs betweensemiconductor layers of the semiconductor heterostructure issubstantially eliminated by fabricating layers with an index ofrefraction that is appropriate for removing the TIR. In general, thesemiconductor heterostructure can include a first semiconductor layerhaving a first index of refraction n1, a second semiconductor layerformed over the first semiconductor layer having an index of refractionn2, a third semiconductor layer formed over the second semiconductorlayer having a third index of refraction n3, wherein n1>n2>n3.

A semiconductor layer of any of the semiconductor heterostructuresdescribed herein can be considered to be transparent to radiation of aparticular wavelength when the layer allows an amount of the radiationradiated at a normal incidence to an interface of the layer to passthere through. For example, a layer can be configured to be transparentto a range of radiation wavelengths corresponding to a peak emissionwavelength for light, such as ultraviolet light or deep ultravioletlight, emitted by a light generating structure (e.g., peak emissionwavelength +/− five nanometers). As used herein, a layer is transparentto radiation if it allows more than approximately five percent of theradiation to pass there through, while a layer can also be considered tobe transparent to radiation if it allows more than approximately tenpercent of the radiation to pass there through. Defining a layer to betransparent to radiation in this manner is intended to cover layers thatare considered transparent and semi-transparent.

A semiconductor layer of any semiconductor heterostructures describedherein can be considered to be reflective when the layer reflects atleast a portion of the relevant electromagnetic radiation (e.g., lighthaving wavelengths close to the peak emission of the light generatingstructure). As used herein, a layer is partially reflective to radiationif it can reflect at least approximately five percent of the radiation,while a layer can also be considered to be partially reflective if itreflects at least thirty percent for radiation of the particularwavelength radiated normally to the surface of the layer. A layer can beconsidered highly reflective to radiation if it reflects at leastseventy percent for radiation of the particular wavelength radiatednormally to the surface of the layer.

The semiconductor heterostructures described herein can be used to formone of a variety of optoelectronic or electronic devices. Examples ofpossible optoelectronic and electronic devices include, but are notlimited to, light emitting devices, light emitting diodes (LEDs),including conventional and super luminescent LEDs, light emitting solidstate lasers, laser diodes (LDs), photodetectors, photodiodes, andhigh-electron mobility transistors (HEMTs), ultraviolet LEDs, andultraviolet LDs. These examples of optoelectronic devices can beconfigured to emit or sense electromagnetic radiation in an activeregion upon application of a bias. The electromagnetic radiation emittedor sensed by these optoelectronic devices can comprise a peak wavelengthwithin any range of wavelengths, including visible light, ultravioletradiation, deep ultraviolet radiation, infrared light, and/or the like.For example, these optoelectronic devices can emit or sense radiationhaving a dominant wavelength within the ultraviolet range ofwavelengths. As an illustration, the dominant wavelength can be within arange of wavelengths of approximately 210 nanometers (nm) toapproximately 350 nm.

The description that follows may use other terminology herein for thepurpose of describing particular embodiments only and is not intended tobe limiting of the disclosure. For example, unless otherwise noted, theterm “set” means one or more (i.e., at least one) and the phrase “anysolution” means any now known or later developed solution. The singularforms “a,” “an,” and “the” are intended to include the plural forms aswell, unless the context clearly indicates otherwise. It will be furtherunderstood that the terms “comprises”, “comprising”, “includes”,“including”, “has”, “have”, and “having” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. Additionally,as used herein, “vertical” is used to reference the growth direction ofthe corresponding structure, while “lateral” is used to reference adirection that is perpendicular to the growth direction. Additionally,spatially relative terms, such as “on,” “below,” “above,” etc., are usedin reference to the orientation shown in the drawings. It is understoodthat embodiments of the invention are not limited to any particularorientation of a device described herein.

The description may also list values of parameters of elements,components, materials, layers, structures, and the like, for the purposeof describing further details of particular embodiments. It isunderstood that, unless otherwise specified, each value is approximateand each range of values included herein is inclusive of the end valuesdefining the range. As used herein, unless otherwise noted, the term“approximately” is inclusive of values within +/− ten percent of thestated value, while the term “substantially” is inclusive of valueswithin +/− five percent of the stated value. Unless otherwise stated,two values are “similar” when the smaller value is within +/−twenty-five percent of the larger value. A value, y, is on the order ofa stated value, x, when the value y satisfies the formula 0.1x≤y≤10x.

As used herein, two materials can have comparable compositions when themolar fractions of the corresponding materials differ by at most tenpercent (five percent in a more specific embodiment). For example,consider two group III nitride materials, Al_(x)In_(y)B_(z)Ga_(1-x-y-z)Nand Al_(x′)In_(y′)B_(z′)Ga_(1-x′-y′-z′)N. The two materials havecomparable compositions when each of the molar fractions x, y, and zdiffers from the corresponding molar fractions x′, y′, and z′ by lessthan ten percent, where the percentage is calculated by taking adifference between the molar fractions and dividing the value by thehigher molar fraction. Similarly, two layers have comparable thicknesseswhen the corresponding thicknesses differ by at most ten percent (fivepercent in a more specific embodiment). Unless otherwise specified, twolayers have similar thicknesses when the respective thicknesses arewithin one nanometer (inclusive) of each other. Similarly, two layershave different thicknesses when the thicknesses differ by more than onenanometer. It is understood that two numbers are on the same order asone another when a ratio of the higher number to the lower number isless than ten.

Compositions of two semiconductor layers can also be evaluated inconjunction with the corresponding band gaps. In this case, as usedherein, compositions of two semiconductor layers are the same when theband gaps of the two semiconductor layers differ by less than thethermal energy unit, kT. The compositions of two semiconductor layersare substantially the same when the band gaps of the two semiconductorlayers differ by less than three times the thermal energy unit, 3 kT. Acomposition of a first semiconductor layer is considered larger than acomposition of a second semiconductor layer when the band gap of thefirst semiconductor layer is larger than the band gap of the secondsemiconductor layer by more than the thermal energy unit, kT. Acomposition of a first semiconductor layer is considered substantiallylarger than a composition of a second semiconductor layer when the bandgap of the first semiconductor layer is larger than the band gap of thesecond semiconductor layer by more than three times the thermal energyunit, 3 kT. Unless otherwise specified, the thermal energy unit isapproximated as 0.026 eV.

Turning to the drawings, FIG. 1 shows a schematic of an optoelectronicdevice 10 formed from a semiconductor heterostructure 11. In a moreparticular embodiment, the optoelectronic device 10 is configured tooperate as an emitting device, e.g., an LED, such as a deep ultravioletlight emitting diode (DUV LED) or a conventional or super luminescentLED. Alternatively, the optoelectronic device 10 can be configured tooperate as a light emitting solid state laser, a laser diode (LD), aphotodetector, a photodiode, or another type of optoelectronic orelectronic (e.g., a HEMT) device. Additional aspects of the inventionare shown and described in conjunction with the optoelectronic device10. However, it is understood that embodiments can be utilized inconjunction with any type of optoelectronic device and/or any type ofgroup III nitride-based device.

When the optoelectronic device 10 operates as an emitting device,application of a bias comparable to the band gap results in the emissionof electromagnetic radiation from an active region 18 of the device 10.The electromagnetic radiation emitted by the optoelectronic device 10can comprise a peak wavelength within any range of wavelengths,including visible light, ultraviolet radiation, DUV radiation, infraredlight, and/or the like. In an embodiment, the optoelectronic device 10is configured to emit radiation having a dominant wavelength within theultraviolet range of wavelengths. In a more specific embodiment, thedominant wavelength is within a range of wavelengths betweenapproximately 210 nanometers and approximately 350 nanometers.

The semiconductor heterostructure 11 of the optoelectronic device 10 caninclude a substrate 12, a buffer layer 14 (e.g., AlN, an AlGaN/AlNsuperlattice, and/or the like) adjacent to the substrate 12, an n-typecontact semiconductor layer 16 (e.g., an electron supply layer, acladding layer and the like) adjacent to the buffer layer 14, and theactive region 18 adjacent to the n-type contact semiconductor layer 16.Furthermore, the heterostructure 11 of the optoelectronic device 10 caninclude a p-type semiconductor layer 20 (e.g., a cladding layer, anelectron blocking layer) adjacent to the active region 18 and a p-typecontact semiconductor layer 22 (e.g., a hole supply layer, claddinglayer, and the like), adjacent to the p-type semiconductor layer 20.

In a more particular illustrative embodiment, the optoelectronic device10 can be a group III-V materials-based device in which some or all ofthe various layers of the semiconductor heterostructure 11 are formed ofelements selected from the group III-V materials system. In a still moreparticular illustrative embodiment, the various layers of thesemiconductor heterostructure 11 of the optoelectronic device 10 can beformed of group III nitride-based materials. Group III nitride materialscomprise one or more group III elements (e.g., boron (B), aluminum (Al),gallium (Ga), and indium (In)) and nitrogen (N), such thatB_(W)Al_(X)Ga_(Y)In_(Z)N, where 0≤W, X, Y, Z≤1, and W+X+Y+Z=1.Illustrative group III nitride materials can include binary, ternary andquaternary alloys such as, AlN, GaN, InN, BN, AlGaN, AlInN, AlBN,AlGaInN, AlGaBN, AlInBN, and AlGaInBN with any molar fraction of groupIII elements.

An illustrative embodiment of a group III nitride-based optoelectronicdevice 10 can include an active region 18 (e.g., a series of alternatingquantum wells and barriers) composed of In_(y)Al_(x)Ga_(1-x-y)N,Ga_(z)In_(y)Al_(x)B_(1-x-y-z)N, an Al_(x)Ga_(1-x)N semiconductor alloy,or the like. Similarly, both the n-type contact layer 16 and the p-typesemiconductor layer 20 can be composed of an In_(y)Al_(x)Ga_(1-x-y)Nalloy, a Ga_(z)In_(y)Al_(x)B_(1-x-y-z)N alloy, or the like. The molarfractions given by x, y, and z can vary between the various layers 16,18, and 20. The substrate 12 can include sapphire, silicon carbide(SiC), silicon (Si), GaN, AlGaN, AlON, LiGaO₂, or another suitablematerial, and the buffer layer 14 can be composed of AlN, an AlGaN/AlNsuperlattice, and/or the like. While further details of various layersare primarily described in conjunction with AlGaN materials, it isunderstood that this material is only illustrative of various materials.To this extent, it is understood that embodiments of such layers alsocan comprise group III nitride materials including boron and/or indium.Additionally, other embodiments can include materials other than groupIII nitride materials, such as other group III-V materials.

As shown with respect to the optoelectronic device 10, a p-type metalcontact 24 can be attached to the p-type contact semiconductor layer 22and a p-type electrode 26 can be attached to the p-type metal contact24. Similarly, an n-type metal contact 28 can be attached to the n-typecontact layer 16 and an n-type electrode 30 can be attached to then-type metal contact 28. The p-type metal contact 24 and the n-typemetal contact 28 can form p-type and n-type ohmic contacts,respectively, to the corresponding layers 22, 16, respectively. It isunderstood that a contact formed between two layers is considered“ohmic” or “conducting” when an overall resistance of the contact is nolarger than the larger of the following two resistances: a contactresistance such that a voltage drop at the contact-semiconductorjunction is no larger than two volts; and a contact resistance at leastfive times smaller than a resistance of a largest resistive element orlayer of a device including the contact.

In an embodiment, the p-type metal contact 24 and/or the n-type metalcontact 28 can comprise several conductive and reflective metal layers,while the n-type electrode 30 and/or the p-type electrode 26 cancomprise highly conductive metal. In an embodiment, the p-type contactsemiconductor layer 22 and/or the p-type electrode 26 can be transparent(e.g., semi-transparent or transparent) to the electromagnetic radiationgenerated by the active region 18. For example, the p-type contactsemiconductor layer 22 and/or the p-type electrode 26 can comprise ashort period superlattice lattice structure, such as a transparentmagnesium (Mg)-doped AlGaN/AlGaN short period superlattice structure(SPSL). Furthermore, the p-type electrode 26 and/or the n-type electrode30 can be reflective of the electromagnetic radiation generated by theactive region 18. In another embodiment, the n-type contact layer 16and/or the n-type electrode 30 can be formed of a short periodsuperlattice, such as an AlGaN SPSL, which is transparent to theelectromagnetic radiation generated by the active region 18.

As further shown with respect to the optoelectronic device 10, thedevice 10 can be mounted to a submount 36 via the electrodes 26, 30 in aflip chip configuration. In this case, the substrate 12 is located onthe top of the optoelectronic device 10. To this extent, the p-typeelectrode 26 and the n-type electrode 30 can both be attached to asubmount 36 via contact pads 32, 34, respectively. The submount 36 canbe formed of aluminum nitride (AlN), silicon carbide (SiC), and/or thelike.

Any of the various layers of the device 10 can comprise a substantiallyuniform composition or a graded composition. For example, a layer cancomprise a graded composition at a heterointerface with another layer.In an embodiment, the p-type semiconductor layer 20 comprises a p-typeelectron blocking layer having a graded composition. The gradedcomposition(s) can be included to, for example, reduce stress, improvecarrier injection, and/or the like. Similarly, a layer can comprise asuperlattice including a plurality of periods, which can be configuredto reduce stress, and/or the like. In this case, the composition and/orwidth of each period can vary periodically or aperiodically from periodto period.

It is understood that the layer configuration of the semiconductorheterostructure 11 of the optoelectronic device 10 described herein isonly illustrative. To this extent, the semiconductor heterostructure 11can include an alternative layer configuration, one or more additionallayers, and/or the like. As a result, while the various layers are shownimmediately adjacent to one another (e.g., contacting one another), itis understood that one or more intermediate layers can be present in thesemiconductor heterostructure 11. For example, an illustrativesemiconductor heterostructure 11 can include an undoped layer betweenthe active region 18 and one or both of the p-type contact semiconductorlayer 22 and the n-type contact semiconductor layer 16 (e.g., anelectron supply layer).

Furthermore, the semiconductor heterostructure 11 can include aDistributive Bragg Reflector (DBR) structure, which can be configured toreflect light of particular wavelength(s), such as those emitted by theactive region 18, thereby enhancing the output power of thedevice/heterostructure. For example, the DBR structure can be locatedbetween the p-type contact semiconductor layer 22 and the active region18. Similarly, the semiconductor heterostructure 11 can include a p-typelayer 20 located between the p-type contact semiconductor layer 22 andthe active region 18. The DBR structure and/or the p-type layer 20 cancomprise any composition based on a desired wavelength of the lightgenerated by the device/heterostructure. In one embodiment, the DBRstructure comprises a Mg, Mn, Be, or Mg+Si-doped p-type composition. Thep-type layer 20 can comprise a p-type AlGaN, AlInGaN, and/or the like.It is understood that the semiconductor heterostructure 11 can includeboth the DBR structure and the p-type layer 20 (which can be locatedbetween the DBR structure and the p-type contact layer 22) or caninclude only one of the DBR structure or the p-type layer 20. In anembodiment, the p-type layer 20 can be included in thedevice/heterostructure in place of an electron blocking layer. Inanother embodiment, the p-type layer 20 can be included between thesecond p-type contact layer 22 and the electron blocking layer.

FIG. 2 shows a band diagram of an illustrative semiconductorheterostructure 38 for an optoelectronic device, such as theoptoelectronic device 10 shown in FIG. 1, and a corresponding example ofa profile of the refractive index for each region within thesemiconductor heterostructure. For purposes of clarity in maintainingcorrespondence with the semiconductor heterostructure 11 of theoptoelectronic device 10 depicted in FIG. 1, like elements in thesemiconductor heterostructure 38 of FIG. 2, as well as theheterostructures of other embodiments, are described with like orcorresponding reference numerals. As illustrated, the semiconductorheterostructure 38 includes the active region 18 sandwiched between then-type contact layer 16 and the p-type layer 20, with the p-type contactlayer 22 immediately adjacent the p-type layer 20. In one embodiment,the n-type contact layer 16 and the p-type layer 20 can include claddinglayers, and the active region 18 can include a laminate of a pluralityof semiconductor layers (e.g., sublayers).

The n-type contact layer 16 can have an index of refraction n1, theactive region 18 including a laminate of a plurality of semiconductorlayers can have an effective index of refraction n2, while the p-typelayer 20 can have a third index of refraction n3. As used herein, anindex of refraction for a layer comprises an average index of refractiontaken over the entire layer. As also used herein, an effective index ofrefraction is the realizable index of refraction for a layer or regionhaving a multiple of sublayers, interlayers, or the like, that isobtained by taking into account each of the respective indexes ofrefraction of all of the sublayers or interlayers that form the layer orregion. As an example, an ellipsometer can be used to measureexperimentally the indexes of refraction of all of the sublayers orinterlayers in a layer or region in order to derive an effective indexof refraction.

In one embodiment, the relationship between the indexes of refraction ofthe n-type contact layer 16, the active region 18 and the p-type layer20 can be described by n1>n2>n3. Having indexes of refractions such thatthe index of refraction n1 of the n-type contact layer 16 is greaterthan the index of refraction n2 of the active region 18 which is greaterthan the index of refraction n3 of the p-type layer 20 eliminates theTIR that can occur between these semiconductor layers. Light emittingdiodes can be fabricated with a quantum efficiency of 70-80% and higher.However, light extraction remains at only a few percent. For lightemitting diodes that emit ultraviolet C radiation, TIR is responsiblefor approximately 90% of the losses. As a result, the improvement oflight extraction can result in a dramatic improvement of the lightemitting diode performance.

In one embodiment, the laminate of the plurality of semiconductorsublayers within the active region 18 can comprise a plurality ofquantum wells and a plurality of barriers, wherein the plurality ofquantum wells alternate or interleave with the plurality of barriers. Inone embodiment, the quantum wells can be significantly thinner than thebarriers. For example, the quantum wells can have a thickness thatcomprises several nanometers (e.g., a thickness less than 5 nm), whilethe barriers can be at least twice as thick (e.g., at least 10 nm ormore). It is understood that the active region 18 can comprise anon-complex structure (e.g., a region that does not include a laminateof semiconductor layers such as quantum wells and barriers). In thisscenario, then it should be appreciated that such an active region willinclude an index of refraction and not an effective index of refraction.

The p-type contact layer 22 formed adjacent the p-type layer 20 can alsoinclude an index of refraction n4. In one embodiment, the index ofrefraction n4 of the p-type contact layer 22 is less than the index ofrefraction n3 of the p-type layer 20, such that n3>n4. To this extent,the relationship between the indexes of refraction of the layers in thesemiconductor heterostructure 38 of FIG. 2 (e.g., the n-type contactlayer 16, the active region 18, the p-type layer 20, and the p-typecontact layer 22) can be described by n1>n2>n3>n4.

In one embodiment, the n-type contact layer 16 can include a claddinglayer (e.g., an n-type cladding layer) formed adjacent the active region18 having a laminate of sublayers that can include quantum wells andbarriers, while the p-type layer 20 can include another cladding layer(e.g., a p-type cladding layer) formed adjacent the opposing side of theactive region 18. In another embodiment, the p-type layer 20 can takethe form of an electron blocking layer, while the p-type contact layer22 can function as the cladding layer (e.g., a p-type cladding layer).As shown in FIG. 2, the p-type contact layer 22 can include a pluralityof interlayers. In one embodiment, the p-type contact layer 22 caninclude a superlattice of interlayers. For example, the superlattice caninclude a p-type superlattice having a plurality of barriers and quantumbarriers.

The indexes of refraction (e.g., n1, n2, n3, n4) of the varioussemiconductor layers in the semiconductor heterostructure 38, and thedesired relationship of the indexes (e.g., n1>n2>n3 and n1>n2>n3>n4) inthe heterostructure can be obtained by selecting appropriate amounts ofthe group III nitride materials (e.g., AlN, GaN, InN, or BN) used in thegroup III nitride layers. For example, the index of refraction of AlN,n(AlN), is less than the index of refraction of GaN, n(GaN), which isless than the index of refraction of InN, n(InN). As a result, todecrease the refractive index of a group III nitride material, a molarfraction of AlN can be increased and/or a molar fraction of InN can bedecreased. In an embodiment, the active region 18 can have a compositionthat results in the active region having an index of refraction that issmaller than the n-type cladding layer 16 and greater than electronblocking layer (e.g., the p-type layer 20). The barriers of the activeregion 18 can comprise a group III nitride material having a low indexof refraction, while the quantum wells can comprise a group III nitridematerial having a high index of refraction.

In one example, the barriers can have a high molar fraction of AlN,while the quantum wells can have a low molar fraction of AlN. To thisextent, the barriers will have low indexes of refraction and the quantumwells will have high indexes of refraction. In this manner, the activeregion 18 can be configured to have an effective index of refractionthat is lower than the index of refraction of the n-type cladding layer16. In one embodiment, the barrier layers within the active region 18can be chosen to have a high molar fraction of AlN to result in aneffective index of refraction that is smaller than the index ofrefraction of the n-type cladding layer 16 that is in proximity to theactive region 18. As used herein, “in proximity” means a region that isat least 20 nm within the distance to the interface between the n-typecladding layer 16 and the active region 18.

It is understood that the molar fraction of AlN within the barriers andthe quantum wells can be chosen to result in the active region 18 havingan effective refractive index that satisfies the relationship betweenthe active region and the n-type cladding layer (e.g., n1>n2). As aresult, there will be an absence of TIR at the interface between theactive region 18 and the n-type cladding layer 16. In this example, theelectron blocking layer (e.g., the p-type layer 20) can have a highmolar fraction of AlN that results in the index of refraction n3 of thislayer being smaller than the index of refraction n2 of the active region18. To this extent, the change in refractive index from n2 to n3 resultsin TIR at the interface between the active region 18 and the electronblocking layer (e.g., the p-type layer 20). With regard to the p-typecontact layer 22 that can be formed adjacent the electron blocking layer(e.g., the p-type layer 20), it can have a low molar fraction of AlN.

An example of a profile of the refractive index for each layer withinthe semiconductor heterostructure 38 is depicted in FIG. 2. Asillustrated, region 40 in the profile that corresponds with the n-typecontact layer 16, shows that the index of refraction n1 for this layercan vary through its thickness. However, in general, and at least inproximity to the interface with the active region 18, the index ofrefraction n1 of the n-type contact layer 16 is larger than the averagedrefractive index (e.g., the effective refractive index) of the activelayer 18 which is depicted in region 42 of the profile in FIG. 2 byreference element 42A. In one embodiment, the refractive index profileof the n-type contact layer 16 can be obtained with a layer thatincludes a molar fraction of InN.

As also shown, region 42 of the refractive index profile furtherdelineates details of the refractive indexes associated with thebarriers and the wells of the active region 18. In this case, the indexof refraction of the barriers are represented by reference element 42B,while the quantum wells are represented by reference element 42C. In oneembodiment, the barriers can have low indexes of refraction and thequantum wells can have high indexes of refraction. As used herein, a lowindex of refraction means an index of refraction below the index ofrefraction of the region 40, while a high index of refraction means anindex of refraction above the index of refraction of the region 40. Inone embodiment, as noted above, quantum wells with a low molar fractionof AlN, and barriers with a high molar fraction of AlN can result in theactive region 18 having an index of refraction that is less than theindex of refraction of the n-type contact layer 16. To this extent,having such molar fractions of AlN in the barriers and the quantum wellscan lead to the active region 18 having an effective index of refraction(e.g., index of refraction 42A) that is less than the index ofrefraction of the n-type contact layer 16. As a result, the interfacebetween the n-type contact layer 16 and the active region 18 will havean absence of TIR.

Region 44 of the profile of the refractive index of the semiconductorheterostructure 38 in FIG. 2 corresponds with the index of refraction ofthe p-type layer 20. In one embodiment, the p-type layer 20 can includea high molar fraction of a group III nitride material with a lowrefractive index. For example, the p-type layer 20 can include a highmolar fraction of AlN. In this manner, as shown in FIG. 2, the p-typelayer 20 can have an index of refraction that is less than the effectiveindex of refraction 42A of the active region 18. As a result, theinterface between the active region 18 and the p-type layer 20 can haveTIR.

In FIG. 2, region 46 of the profile of the refractive index representsthe effective index of refraction of the p-type contact layer 22 in thesemiconductor heterostructure 38. As noted above, the p-type contactlayer 22 can include a p-type superlattice with interlayers or a p-typelayer. In either case, a certain composition of group III nitridematerials can be used to obtain a desired index of refraction for thep-type contact layer 22. In one embodiment, the p-type contact layer 22can have a low molar fraction of AlN or GaN. A low molar fraction of GaNand a high molar fraction of AlN in the p-type contact layer 22 canresult in this layer having an index of refraction that is less than theeffective index of refraction of the active region 18 and of the p-typelayer 20 as shown in FIG. 2.

In order for the semiconductor heterostructure 38 to provide improvedlight emission, it is preferable that most of the semiconductor layers(e.g., the n-type contact layer 16, the active region 18, the p-typelayer 20, and the p-type contact layer 22) in the heterostructure bepartially transparent to the emitting or sensing radiation. FIG. 3 showsan illustrative view of a band diagram of another semiconductorheterostructure 38 along with an example of a profile of the absorptionvalues of the semiconductor layers within the semiconductorheterostructure 38. The band diagram of FIG. 3 shows that thesemiconductor heterostructure 38 can operate at a wavelength of emission(photon energy) corresponding to the energy separation G1 of the energylevels within the sublayers (e.g., the quantum wells and barriers) ofthe active region 18. The average band gap G2 of the p-type contactlayer 22, which as mentioned previously can include a superlattice, canbe selected to be greater than G1 to ensure that the superlattice issufficiently transparent to the emitted (absorbed) target wavelength ofthe energy G1 from the active region 18.

In one embodiment, in which the p-type contact layer 22 is asuperlattice having quantum wells alternating with barriers, the energyG2 can correspond to the minimum value of average band gaps computedover each period of the superlattice, wherein each period includes onebarrier and one quantum well. The thicknesses of either the quantumwells or the barriers, or both, of the p-type contact layer 22 can varyas demonstrated by reference element 48 which happens to show thevariable thicknesses of the quantum wells. The thicknesses of thequantum wells and/or barriers within the p-type superlattice 22 canrange from a few angstroms to a few nanometers, and, in some cases, canbe on the order of 10 nanometers.

FIG. 3 further shows that the average band gap of the n-type contactlayer 16 (e.g., an n-type cladding layer) can also be larger than G1. Asa result, the n-type contact layer 16 can have significant transparency.As used herein, significant transparency means at least 50% transparencyto the target wavelength emitted normal to the interface of the layer.

In an embodiment of FIG. 3, the relationship between the indexes ofrefraction of the layers in the semiconductor heterostructure 38 (e.g.,the n-type contact layer 16, the active region 18, the p-type layer 20,and the p-type contact layer 22) can be described by n1>n2>n3<n4. Such aconfiguration can represent a compromise between a low index ofrefraction for the p-type contact layer 22 and a high conductivity ofthe p-type contact layer 22. Since an increase in the AlN molar fractionresults in lower conductivity, it may not be desirable to include asufficient amount of AlN to make n4<n3. To this extent, in FIG. 3 therefractive index of the p-type contact layer 22 may not be low enoughsuch that n4>n3, but the p-type contact layer 22 can be transparent foremitted light. Inclusion of a reflecting metal p-type contact adjacentto the p-type contact layer 22, can allow light to be reflected backthrough the p-type contact layer 22 and extracted from the n-type (e.g.,sapphire) side of the LED with minimum absorption within the p-typecontact layer 22.

The band gap diagram of FIG. 3 further illustrates several differentcharacteristics that the various layers of the semiconductorheterostructure 38 can have that contribute to it having improvedemission. For example, FIG. 3 shows that the band gap of both the p-typelayer 20 and the barriers within the superlattice of the p-type contact22 can be wider than the band gap of the quantum wells within the activeregion 18. In an embodiment, the average band gap over the p-typesuperlattice 22 can be wider than the band gap of the quantum wellswithin the active region 18. In an embodiment, the average band gap overthe p-type superlattice 22 can be wider than the photon energycorresponding to the target radiation.

The band gap of the semiconductor heterostructure 38 can bedistinguished with still more characteristics. For example, the p-typecontact layer 22 (e.g., the p-type superlattice layer) can have aneffective band gap that is higher than the band gap of a sublayer withinthe laminate of sublayers (e.g., quantum wells) of the active region 18having the lowest band gap of the sublayers. In one embodiment, at leastone sublayer of the laminate of sublayers (e.g., quantum wells) of theactive region 18 can have a band gap that is less than the band gap ofthe n-type contact layer 16, the other sublayers of the active region18, and the p-type layer 20.

In one embodiment, sublayers in the laminate of sublayers (e.g.,barriers) of the active region 18 can have a wider band gap than theband gap of the n-type contact layer 16 at a location that is in theimmediate vicinity of an interface between the n-type contact layer 16and the active region. As used herein, in the immediate vicinity of theinterface means a distance being at most 20 nm. FIG. 3 also shows thatthe p-type layer 20 can comprise a variable band gap. FIG. 3 furthersshows that the p-type layer 20 can have a maximum band gap that islarger than the band gap of any sublayer within the laminate ofsublayers (e.g., barriers and quantum wells) of the active region 18. Inaddition, FIG. 3 shows that any sublayer within the laminate ofsublayers (e.g., barriers and quantum wells) of the active region 18 cancomprise a band gap that is lower than a minimum band gap of the n-typecontact layer 16.

As mentioned above, most of the semiconductor layers (e.g., the n-typecontact layer 16, the active region 18, the p-type layer 20 and thep-type contact layer 22) of the semiconductor heterostructure 38 can bepartially transparent to the emitting or sensing radiation for purposesof improving light emission. FIG. 3 illustrates an example of a profileof the absorption values for the semiconductor layers within thesemiconductor heterostructure 38. The absorption of the layers to atarget radiation is represented in FIG. 3 in arbitrary units. As shownin FIG. 3, region 50, which represents the absorption of the targetradiation within the n-type contact layer 16, has a substantiallyuniform, low absorption of the radiation. As used herein, low absorptionrefers to sub-bandgap absorption (10-1000 cm⁻¹).

The most absorption that occurs in the semiconductor heterostructure isin the active region 18 which is depicted in region 52 of the absorptionprofile. In particular, the quantum wells of the active region, whichare shown as region 52A, have the most absorption, while the barriershave the lowest absorption as depicted by region 52B. The mix of levelsof absorption that occurs in the quantum wells and the barriers resultsin the active region having an average or effective absorption shown by52C. In one embodiment, the average absorption of the active region 18can be selected to be similar to the absorption of the n-type contactlayer 16 (e.g., the n-type cladding layer) and the p-type layer 20(e.g., the p-type electron blocking layer) which is represented in theabsorption profile of FIG. 3 by region 54. As used herein, similarabsorption means at least 50% transparency for the wavelength of thetarget radiation. FIG. 3 also shows that the p-type contact layer 22 canhave a substantially uniform, low absorption of the target radiation asshown by region 56. FIG. 3 also shows that an absorption of then-contact layer 16 and the p-type layer 20 can be higher than theabsorption of the sublayer (e.g., quantum well) within the active regionhaving a lowest band gap.

In order to further improve control of the indexes of refraction of thesemiconductor layers (e.g., the n-type contact layer 16, the activeregion 18, the p-type layer 20 and the p-type contact layer 22) in thesemiconductor heterostructure 38, additional semiconductor materials canbe added to the semiconductor layers to attain desired indexes ofrefraction by enhancing electrical and optical properties of the layers.In one embodiment, boron nitride (BN) can be added to any of thesemiconductor layers of the semiconductor heterostructure 38. Addingsmaller fraction amounts of BN to a group III nitride-basedsemiconductor layer in the semiconductor heterostructure will notsignificantly affect the band gap of the semiconductor layer, but suchadditions can have notable decreases in the index of refraction of thesemiconductor layers. For example, a slight addition of BN to AlN andGaN layers can result in significant decreases in the indexes ofrefraction of the semiconductor layers.

By way of illustration, FIG. 4A shows that a small fraction of BN doesnot significantly affect the band gap of a semiconductorheterostructure. In particular, FIG. 4A shows that the addition of BN atthe expense of an AlN molar fraction does not significantly affect theband gap of a semiconductor layer in a semiconductor heterostructure.FIG. 4A further shows that even an addition of 17% of BN does notsignificantly alter the band gap of the material. Interestingly, datafrom FIGS. 4A and 4B also show that a small addition of BN to AlN canincrease the band gap of the material.

While the addition of BN does not significantly affect band gapcharacteristics of semiconductor layers, FIGS. 5A and 5B show that evena slight addition of BN to both AlN and GaN results in significantdecreases in the index of refraction of the semiconductor layers. Inparticular, an addition of BN to an active region and an electronblocking layer can create a significant change in the refractive indexof the layers. In one embodiment, BN can be added to the active region18 of the semiconductor heterostructure 38. For example, in the case inwhich the active region 18 includes barriers and quantum wells, a molarfraction of BN can be added to at least one of: the barriers or thequantum wells, of the active region. In one embodiment, a molar fractionof BN can be added to all of the barriers and the quantum wells of theactive region 18. In general, the amount of BN that is utilized caninclude a molar fraction that preserves the targeted radiation specifiedfor the optoelectronic device incorporating the semiconductorheterostructure 38. Namely, the molar fraction of BN that is selectedcan preserve the target photon energy G1, which can be further helped byslightly adjusting the thickness of the quantum well layers.

The molar fraction of BN can be added to other layers of thesemiconductor heterostructure 38 and is not meant to be limited to usewith the active region 18. For example, a molar fraction of BN can beadded to the p-type layer 20 and/or the p-type contact layer 22. In oneembodiment, a molar fraction of BN can be added to the p-type layer 20that functions as an electron blocking layer or as a cladding layer.Similarly, the molar fraction of BN can be added to the p-type contactlayer 22 that functions as a cladding layer. For example, in thescenario in which the p-type contact layer 22 includes a p-type layer ofinterlayers, a molar fraction of BN can be added to the interlayers. Inan embodiment in which the p-type contact layer 22 includes a p-typesuperlattice of barriers and quantum wells, a molar fraction of BN canbe added to least one of the barriers and/or the quantum wells.

It is understood that the BN molar fraction can be added in variousways. In one embodiment, the BN can be added as interlayers comprising asingle atomic plane or a few atomic planes. In an embodiment, the BNmolar fraction can be varied with the thickness of the layer to which itis added.

FIG. 6 illustrates an example of an effect that the addition of BN canhave on the index of refraction profile of the semiconductor layerswithin the semiconductor heterostructure 38. In particular, FIG. 6 showsthe effect that the addition of BN can have on the index of refractionprofile of the semiconductor layers within the semiconductorheterostructure 38 in relation to the profile depicted in FIG. 2. Theindex of refraction profile of the semiconductor layers with theaddition of BN is shown in FIG. 6 by a solid-lined profile, while theindex of refraction profile of the layers without BN is represented bythe dash-lined profile.

As shown in FIG. 6, the addition of BN will have essentially no effecton the index of refraction of the n-type contact layer 16, however, theaddition of BN will have an effect on the index refraction of the otherlayers in the semiconductor heterostructure 38. In particular, in theactive region 18, the addition of BN will result in an effective indexof refraction that is less than the effective index of refraction of theactive region that does not contain BN, as shown by the region 42A inFIG. 6. This change in the effective index of refraction of the activeregion 18 arises because the addition of BN to the active region resultsin the barriers and the quantum wells having a lower index of refractionas shown by regions 42B and 42C, respectively, in comparison to thebarriers and quantum wells of the active region that is without BN.

As further shown in FIG. 6, the addition of BN does not have much of aneffect on the index of refraction of the p-type layer 20, but it doeshave a much larger effect on the index of the p-type contact layer 22like with the active region 18. In particular, FIG. 6 shows that theindex of refraction for the p-type layer 20 is slightly lower with theaddition of BN as shown in region 44, by the smaller change from theindex of refraction that represents the p-type layer 20 without BN. Onthe other hand, the addition of BN to the p-type contact layer 22 (e.g.,p-type superlattice, interlayer) can result in an index of refractionthat is significantly less than the index of refraction of the p-typecontact layer 22 without BN.

Essentially, FIG. 6 shows that the addition of BN to the semiconductorheterostructure 38 will have a greater impact on the indexes ofrefraction of the active region 18 and the p-type contact layer 22, andless of an impact on the n-type contact layer 16 and the p-type layer20. As a result, the changes or differences in the profile of the indexof refraction for the active region 18 and the p-type contact layer 22that occur with the addition of BN will be significant, especially incomparison to the n-type contact layer 16 and the p-type layer 20.

The control and improvement of the indexes of refraction of thesemiconductor layers of the semiconductor heterostructure 38 are notlimited to the addition of BN, but can include adding other materialsand compounds such as boron (B) and/or indium nitride (InN). Forexample, an interlayer of B can be added to the p-type contact layer 22.In one embodiment, at least one sublayer within the laminate ofsublayers (e.g., barriers and quantum wells) of the active region 18 cancomprises an interlayer of boron. Similarly, at least one layer of thep-type contact layer 22 that is in the form of a p-type superlattice caninclude an interlayer of boron, such as the barriers. In one embodiment,the n-type contact layer 16 can comprise a non-zero molar fraction ofInN.

The emission of the semiconductor heterostructure 38 can also beimproved by nano-patterning one or more of the semiconductor layers thatform the heterostructure. As used herein, nano-patterning means adding aregular or irregular pattern to a surface having a roughness scale onthe order of 100 nanometers. In general, the nano-patterning can beimplemented with the semiconductor heterostructure 38 through the use ofa nano-layer such as anodized aluminum nitride (AlN). For example, theactive region 18 of the semiconductor heterostructure 38 can benano-patterned. The p-type layer 20 (e.g., an electron blocking layer)is another layer of the semiconductor heterostructure 38 that can benano-patterned. It is understood that the other layers of thesemiconductor heterostructure 38 such as the n-type contact 16 and thep-contact layer 22 can also be nano-patterned.

FIGS. 7A and 7B show a cross-sectional view and a top view,respectively, of an illustrative semiconductor heterostructure 58 havingnano-patterned domains 60 formed in the active region 18 according to anembodiment. In a scenario in which the active region 18 contains alaminate of sublayers such as for example, barriers and quantum wells,each nano-domain 60 can include stacked semiconductor sublayers (e.g.,the barriers and quantum wells). The nano-domains 60 of stackedsemiconductor sublayers can form a set of domains in the active region18.

As shown in FIGS. 7A-7B, the nano-domains 60 can be separated fromadjacent nano domains of stacked semiconductor sublayers by apredetermined spacing. In one embodiment, an insulating layer 62,including but not limited to, AAO, SiO₂, Al₂O₃, CaF₂, MgF₂, and/or thelike, can be formed in the spacings separating adjacent nano domains 60of stacked semiconductor sublayers. To this extent, the layers ofinsulating material formed between each of the spacings of the nanodomains 60 collectively form a set of nano domains of insulatingmaterial within the active region 18. In one embodiment, the set of nanodomains of insulating material can have an index of refraction that isless than the index of refraction of the nano-domains 60 of stackedsemiconductor sublayers. The index of refraction of the nano domains ofinsulating material 62 combined with the index of refraction of thenano-domains 60 of stacked semiconductor sublayers to yield an effectiveindex of refraction for the active region 18. It is understood that theeffective index of refraction will depend on the average coverage of theinsulating layer 62.

In an embodiment, the nano-patterned domains 60 formed in the activeregion 18 form a photonic crystal, which can have resonance emission ata certain wavelength of radiation. Additionally, a photonic crystal canlimit light transport laterally. By limiting lateral transport,absorption outside the active area 18, for example, at the mesa edgesnot covered with p-ohmic metal, can be limited. Without this structure,the active area 18 can work like a waveguide that spreads lightlaterally with strong absorption.

The emission of the semiconductor heterostructure 38 can also beimproved by utilizing any of a multitude of other modalities. Forexample, any of the semiconductor layers (e.g., the n-type contact layer16, the active region 18, the p-type layer 20, and the p-type contact22) can be formed with a roughness surface at the interface withadjacent layers. Roughness at an interface allows for partialalleviation of the light trapping by providing additional surfacesthrough which light can escape without totally internally reflectingfrom the interface. In one embodiment, an interface between the n-typecontact layer 16 (i.e., the n-type cladding layer) and the active region18 can include a roughness surface. In one embodiment, the roughnesssurface is larger than a wavelength associated with a target radiationthat the heterostructure is designed for.

Another example of a modality that can be used to improve the emissionof the semiconductor heterostructure 38 includes applying ananti-reflective coating to at least one of the semiconductor layers ofthe heterostructure. In one embodiment, an anti-reflective coating canbe applied at a side of the n-contact layer 16 (e.g., the n-typecladding layer) that is opposite to the interface with the active region18.

In one embodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more of optoelectronicdevices and semiconductor heterostructures having any of thesemiconductor structures described herein. To this extent, FIG. 8 showsan illustrative flow diagram for fabricating a circuit 1260 according toan embodiment. Initially, a user can utilize a device design system 1100to generate a device design 1120 for an optoelectronic semiconductordevice as described herein. The device design 1120 can comprise programcode, which can be used by a device fabrication system 1140 to generatea set of physical devices 1160 according to the features defined by thedevice design 1120. Similarly, the device design 1120 can be provided toa circuit design system 1200 (e.g., as an available component for use incircuits), which a user can utilize to generate a circuit design 1220(e.g., by connecting one or more inputs and outputs to various devicesincluded in a circuit). The circuit design 1220 can comprise programcode that includes a device designed as described herein. In any event,the circuit design 1220 and/or one or more physical devices 1160 can beprovided to a circuit fabrication system 1240, which can generate aphysical circuit 1260 according to the circuit design 1220. The physicalcircuit 1260 can include one or more devices 1160 designed as describedherein.

In another embodiment, the invention provides a device design system1100 for designing and/or a device fabrication system 1140 forfabricating a semiconductor device 1160 as described herein. In thiscase, the system 1100, 1140 can comprise a general purpose computingdevice, which is programmed to implement a method of designing and/orfabricating the semiconductor device 1160 as described herein.Similarly, an embodiment of the invention provides a circuit designsystem 1200 for designing and/or a circuit fabrication system 1240 forfabricating a circuit 1260 that includes at least one device 1160designed and/or fabricated as described herein. In this case, the system1200, 1240 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thecircuit 1260 including at least one semiconductor device 1160 asdescribed herein. In either case, the corresponding fabrication system1140, 1240, can include a robotic arm and/or electromagnet, which can beutilized as part of the fabrication process as described herein.

In still another embodiment, the invention provides a computer programfixed in at least one computer-readable medium, which when executed,enables a computer system to implement a method of designing and/orfabricating a semiconductor device as described herein. For example, thecomputer program can enable the device design system 1100 to generatethe device design 1120 as described herein. To this extent, thecomputer-readable medium includes program code, which implements some orall of a process described herein when executed by the computer system.It is understood that the term “computer-readable medium” comprises oneor more of any type of tangible medium of expression, now known or laterdeveloped, from which a stored copy of the program code can beperceived, reproduced, or otherwise communicated by a computing device.

In another embodiment, the invention provides a method of providing acopy of program code, which implements some or all of a processdescribed herein when executed by a computer system. In this case, acomputer system can process a copy of the program code to generate andtransmit, for reception at a second, distinct location, a set of datasignals that has one or more of its characteristics set and/or changedin such a manner as to encode a copy of the program code in the set ofdata signals. Similarly, an embodiment of the invention provides amethod of acquiring a copy of program code that implements some or allof a process described herein, which includes a computer systemreceiving the set of data signals described herein, and translating theset of data signals into a copy of the computer program fixed in atleast one computer-readable medium. In either case, the set of datasignals can be transmitted/received using any type of communicationslink.

In still another embodiment, the invention provides a method ofgenerating a device design system 1100 for designing and/or a devicefabrication system 1140 for fabricating an optoelectronic semiconductordevice as described herein. In this case, a computer system can beobtained (e.g., created, maintained, made available, etc.) and one ormore components for performing a process described herein can beobtained (e.g., created, purchased, used, modified, etc.) and deployedto the computer system. To this extent, the deployment can comprise oneor more of: (1) installing program code on a computing device; (2)adding one or more computing and/or I/O devices to the computer system;(3) incorporating and/or modifying the computer system to enable it toperform a process described herein; and/or the like.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A semiconductor heterostructure, comprising: afirst semiconductor layer having a first index of refraction n1; asecond semiconductor layer formed over the first semiconductor layer,the second semiconductor layer including a laminate of a plurality ofsemiconductor sublayers having an effective index of refraction n2; anda third semiconductor layer formed over the second semiconductor layer,the third semiconductor layer having a third index of refraction n3,wherein n1>n2>n3; and wherein the laminate of the plurality ofsemiconductor sublayers of the second semiconductor layer has an averageabsorption to a target radiation that is higher than an averageabsorption to the target radiation of the first semiconductor layer andan average absorption to the target radiation of the third semiconductorlayer.
 2. The semiconductor heterostructure of claim 1, wherein thefirst index of refraction n1 varies through a thickness of the firstsemiconductor layer.
 3. The semiconductor heterostructure of claim 1,wherein the laminate of the plurality of semiconductor sublayerscomprises a plurality of quantum wells and a plurality of barriers,wherein the plurality of quantum wells alternate with the plurality ofbarriers, wherein the plurality of barriers have low indexes ofrefraction that are below the index of refraction n1 and the pluralityof quantum wells have high indexes of refraction that are above theindex of refraction n1.
 4. The semiconductor heterostructure of claim 3,wherein at least one of: the plurality of quantum wells or the pluralityof barriers comprises a molar fraction of boron nitride.
 5. Thesemiconductor heterostructure of claim 4, wherein the molar fraction ofboron nitride comprises interlayers of boron nitride, wherein theinterlayers comprise at least one of a single atomic plane or a fewatomic planes.
 6. The semiconductor heterostructure of claim 1, furthercomprising a fourth semiconductor layer formed over the thirdsemiconductor layer, the fourth semiconductor layer having a fourthindex of refraction n4, wherein n3>n4.
 7. The semiconductorheterostructure of claim 6, wherein the fourth semiconductor layercomprises a p-type superlattice with boron interlayers.
 8. Thesemiconductor heterostructure of claim 6, wherein the fourthsemiconductor layer comprises an effective band gap that is higher thanthe band gap of a sublayer within the laminate of the plurality ofsublayers of the second semiconductor layer having a lowest band gap ofthe sublayers.
 9. The semiconductor heterostructure of claim 1, whereinthe laminate of the plurality of semiconductor sublayers of the secondsemiconductor layer has an average absorption to the target radiationthat is similar to the average absorption of the first semiconductorlayer and the third semiconductor layer.
 10. The semiconductorheterostructure of claim 1, wherein the second semiconductor layercomprises a nano-patterned layer, wherein the laminate of the pluralityof semiconductor sublayers comprises a set of nano domains of stackedsemiconductor sublayers, each nano domain of stacked semiconductorsublayers separated from adjacent nano domains of stacked semiconductorsublayers by a predetermined spacing; and a set of nano domains ofinsulating material, each nano domain of insulating material formed inone of the spacings separating adjacent nano domains of stackedsemiconductor sublayers, wherein the set of nano domains of insulatingmaterial has an index of refraction that is less than the index ofrefraction of the set of stacked semiconductor sublayers.
 11. Asemiconductor heterostructure, comprising: a first cladding layer havinga first index of refraction n1; an active region formed over the firstcladding layer, the active region including a plurality of semiconductorsublayers having an effective index of refraction n2; a second claddinglayer formed over the active region, wherein the second cladding layeris in direct and physical contact with the active region, the secondcladding layer having a third index of refraction n3 corresponding to amaximum index of refraction within the second cladding layer, whereinn1>n2>n3; and a semiconductor layer formed over the second claddinglayer, the semiconductor layer having a fourth index of refraction n4,wherein n3>n4.
 12. The semiconductor heterostructure of claim 11,wherein at least one sublayer from the plurality of semiconductorsublayers of the active region has a band gap that is less than the bandgaps of: the first cladding layer and the second cladding layer.
 13. Thesemiconductor heterostructure of claim 11, wherein a set of sublayerswithin the plurality of semiconductor sublayers of the active region hasa wider band gap than the band gap of the first cladding layer at alocation that is in the immediate vicinity of an interface between thefirst cladding layer and the active region.
 14. The semiconductorheterostructure of claim 11, wherein any one sublayer of the pluralityof semiconductor sublayers of the active region comprises a band gapthat is lower than a minimum band gap of the first cladding layer. 15.The semiconductor heterostructure of claim 11, wherein the secondcladding layer comprises a variable band gap.
 16. The semiconductorheterostructure of claim 11, wherein the second cladding layer comprisesa maximum band gap that is larger than the band gap of any one sublayerof the plurality of semiconductor sublayers of the active region. 17.The semiconductor heterostructure of claim 11, wherein the semiconductorlayer formed over the second cladding layer comprises a superlattice ofinterlayers.
 18. The semiconductor heterostructure of claim 11, whereinthe semiconductor layer comprises a molar fraction of boron.
 19. Thesemiconductor heterostructure of claim 11, wherein the semiconductorlayer comprises a plurality of interlayers each including a molarfraction of boron.
 20. An optoelectronic device, comprising: asemiconductor heterostructure, including: an n-type cladding layerhaving a first index of refraction n1 and an average band gap; an activeregion formed over the n-type cladding layer, the active regionincluding a plurality of quantum wells alternating with a plurality ofbarriers, wherein the plurality of quantum wells and the plurality ofbarriers have an effective index of refraction n2, the active regionhaving an average band gap G1, wherein the average band gap of then-type cladding layer is greater than the average band gap G1 of theactive region; an electron blocking layer formed over the active region,the electron blocking layer having a third index of refraction n3; and ap-type layer having an average band gap G2 formed over the electronblocking layer, wherein the average band gap G2 of the p-type layer isgreater than the average band gap G1 of the active region, the p-typelayer having a fourth index of refraction n4, wherein n1>n2>n3>n4, andwherein the p-type layer includes a molar fraction of boron.